DESCRIPTION (APPLICANT ABSTRACT): Concerns over patient safety in hospitals have focused attention on the causes of adverse events and medical errors. One possible cause leading to adverse events is the general stress placed on medical care during times of workload pressure at times of heightened patient activity. Recent media attention on the practice of diverting patients when hospitals become over-crowded adds to concerns over unintended consequences of increased competition in healthcare. This project will test whether day to day variations in hospital occupancy, case complexity, and other measures of patient activity are related to the rate of adverse events, controlling for nurse staffing levels. In addition, because monitoring adverse events is expensive, the study will test alternative methods of identifying adverse events using electronic medical record systems. The study will be performed at four hospitals, three in Massachusetts and one in Utah, representing a mix of teaching status and urban/suburban locations. The study has two aims. 1) We will examine the association between hospital activity/workload and rates of adverse events, with control for staffing levels. As a sensitivity analysis, we will measure adverse drug events (ADES) using automated ADE systems at two of the study hospitals and repeat the analyses. The statistical analyses will use stratified time-dependent Cox regression models to estimate the probability of adverse events, with the patient day as the unit of analysis. 11) We will develop and test the feasibility of a new method of screening for adverse events using a computerized hospital information system which is available at one of the study hospitals. Aim 1 will have implications for hospitals and policy makers deciding whether to limit workload, or else reengineer processes during times of high workload pressure, in order to improve patient safety. A second potential outcome will be to make available cost-effective screening tools for monitoring adverse events and errors.